In a carriage printer, such as an inkjet carriage printer, a printhead is mounted in a carriage that is moved back and forth across a region of printing. To print an image on a sheet of paper or other print medium, the print medium is advanced a given nominal distance along a media advance direction and then stopped. Media advance is typically done by a roller and the nominal distance is typically monitored indirectly by a rotary encoder. While the print medium is stopped and supported on a platen, the printhead carriage is moved in a direction that is substantially perpendicular to the media advance direction as marks are controllably made by marking elements on the print medium—for example by ejecting drops from an inkjet printhead. The position of the carriage and the printhead relative to the print medium is precisely monitored directly, typically using a linear encoder. After the carriage has printed a swath of the image while traversing the print medium, the print medium is advanced; the carriage direction of motion is reversed; and the image is formed swath by swath.
In order to produce high quality images, it is helpful to provide information to printer controller electronics regarding the printing side of the recording medium, which can include whether it is a glossy or matte-finish paper. Such information can be used to select a print mode that will provide an optimal amount of ink in an optimal number of printing passes in order to provide a high quality image on the identified media type. It is well-known to provide identifying marks or indicia, such as a bar code, on a non-printing side of the recording medium to distinguish different types of recording media. It is also well known to use a sensor in the printer to scan the indicia and thereby identify the recording medium and provide that information to the printer control electronics. U.S. Pat. No. 7,120,272, for example includes a sensor that makes sequential spatial measurements of a moving media that contains repeated indicia to determine a repeat frequency and repeat distance of the indicia. The repeat distance is then compared against known values to determine the type of media present.
U.S. Pat. No. 8,033,628 discloses the use of a backside media sensor to read a manufacturer's code for identifying the media type. In this approach light from a light source is reflected from the backside of the media and received in a photosensor while the print media is being advanced past the photosensor. A source of unreliability in interpreting the signals is that media can slip during its advance past the photosensor.
Identification of media type by using transmitted light to detect a manufacturer's code, such as a bar code, has been disclosed in U.S. Pat. No. 8,282,183. In this approach, an optical sensor is provided on the carriage and a light source is provided on an opposite side of the media path so that the optical sensor on the carriage detects light transmitted through the print medium. This approach works well, and is especially useful in so-called L-path printers, where the print side of the print medium faces outward in the media input tray. However, it is found that if a viewing hole for the optical sensor is provided at the bottom of the carriage, accumulated ink at the bottom of the carriage can obstruct the viewing hole in some conditions after long-term usage of the printer.
Consequently, a need exists for a way to keep the optical path to a carriage-mounted sensor unobstructed by ink accumulation.